This invention relates to the preparation of ceramic materials with reduced oxygen levels from polycarbosilanes by the pyrolysis of a mixture of a polycarbosilane, a hydrosilylation catalyst, and an unsaturated compound selected from the group consisting of reactive diolefins, reactive alkynes, polyolefins, vinylsilanes, and unsaturated siloxanes where the mixture is rendered infusible prior to pyrolysis by heating to relatively low temperatures in an inert atmosphere. This invention is especially well suited for the production of ceramic fibers from polycarbosilanes.
Generally, in preparing a shaped ceramic article such as a fiber from a preceramic polymer by pyrolysis at elevated temperatures, it is necessary, prior to pyrolysis, to render the shaped article infusible. Otherwise the shaped article will melt upon pyrolysis and thus the desired shape will be destroyed. The most common method of rendering the shaped article infusible has been an oxidation treatment. This method has the disadvantage of incorporating large amounts of oxygen in the resulting ceramic article. For example, standard grade Nicalon ceramic fibers, prepared from polycarbosilanes by Nippon Carbon Company Ltd, Tokyo, Japan, normally contain about 10-15 weight percent oxygen. High oxygen content results in decreased thermal stability of the ceramic materials at elevated temperatures.
Ceramic materials prepared from polycarbosilanes are known in the art. Verbeek et al. in German Application Publication No. 2,236,078, which is hereby incorporated by reference, prepared ceramic materials by firing a polycarbosilane prepared by the pyrolysis of monosilanes at elevated temperatures in an inert atmosphere. Linear, high molecular weight polymers such as polyethylene oxide, polyisobutylene, polymethylmethacrylate, polyisoprene, and polystyrene were reported to improve the fiber spinning characteristics of the polycarbosilanes. The polycarbosilane fibers were rendered infusible prior to pyrolysis by either thermal, oxidation, sulfidation, or hydrolysis treatment. The ceramic fibers were reported to contain between 0 and 30 weight percent oxygen but no details were given.
Yajima et al. in U.S. Pat. Nos. 4,052,430 (Oct. 4, 1977) and 4,100,233 (July 11, 1978), which are both hereby incorporated by reference, prepared ceramic materials by the pyrolysis of polycarbosilanes in an inert atmosphere or in a vacuum at an elevated temperature. The polycarbosilanes were prepared by thermally decomposing and polycondensing polysilanes. Polycarbosilane fibers were treated for 2-48 hours at 350.degree.-800.degree. C. under vacuum prior to pyrolysis to remove low molecular weight material. In some cases the fibers were first exposed to an oxidizing atmosphere at 50.degree.-400.degree. C. to form an oxide layer on the fibers and then treated under vacuum at 350.degree.-800.degree. C. The oxygen content of the resulting ceramic fibers was not reported.
Yajima et al. in U.S. Pat. Nos. 4,220,600 (Sept. 2, 1980) and 4,283,376 (Aug. 11, 1981), which are both hereby incorporated by reference, prepared ceramic materials by the pyrolysis of polycarbosilanes partly containing siloxane bonds at an elevated temperature under an inert atmosphere or a vacuum. These polycarbosilanes were prepared by heating polysilanes in the presence of about 0.01 to 15 weight percent of a polyborosiloxane in an inert atmosphere. Polycarbosilane fibers were rendered infusible prior to pyrolysis by either treatment with an oxidizing atmosphere at about 50.degree.-400.degree. C. to form an oxide layer on the fiber surface or by irradiation with gamma-rays or an electron beam under an oxidizing or non-oxidizing atmosphere. The oxygen content of the resulting ceramic fibers were in the range of 0.01 to 10 weight percent by chemical analysis. Oxygen in the form of silica could be further removed from the ceramic fiber by treatment in a hydrofluoric acid solution.
Iwai et al. in U.S. Pat. No. 4,377,677 (Mar. 22, 1983), which is hereby incorporated by reference, also produced ceramic materials by the pyrolysis of polycarbosilanes at elevated temperatures under an inert atmosphere or vacuum. The polycarbosilanes of Iwai were prepared by heating a polysilane at 50.degree.-600.degree. C. in an inert gas, distilling out a low molecular weight polycarbosilane fraction and then polymerizing the distilled fraction at 250.degree. to 500.degree. C. in an inert atmosphere. Polycarbosilane fibers were rendered infusible prior to pyrolysis by heating at relatively low temperatures in air. The oxygen content of the resulting ceramic fibers was not reported.
Schilling et al. in U.S. Pat. No. 4,414,403 (Nov. 8, 1983), which is hereby incorporated by reference, produced ceramic material by the pyrolysis of branched polycarbosilanes at elevated temperatures under an inert atmosphere or vacuum. The branched polycarbosilanes were prepared by reacting monosilanes with an active metal in an inert solvent at elevated temperatures where at least some of the monosilanes contained vinyl groups or halomethyl groups capable of forming branching during the polymerization. Methods of rendering the material infusible were not discussed.
Yajima et al., J. Mat. Sci., 13, 2569 (1978), Yajima, Bull. Amer. Ceram. Soc., 62, 893 (1983), and Hasegawa et al., J. Mat. Sci., 18, 3633 (1983) also discuss polycarbosilanes which are useful as preceramic polymers for preparing silicon carbide ceramics. In the Bull. Amer. Ceram. Soc. article Yajima prepared ceramic fibers from polycarbosilanes which had been rendered infusible prior to pyrolysis by heating in air at 190.degree. C. The resulting fibers contained 15.5 weight percent oxygen most of which was thought to be incorporated into the fiber during the curing step.
What has been discovered is a new method of rendering preceramic polycarbosilane polymers infusible prior to pyrolysis which results in a significantly reduced oxygen content in the ceramic materials produced from the pyrolysis of these infusible polycarbosilane polymers. This method represents a significant advance in the art of preparing ceramic materials or articles, especially in the art of preparing ceramic fibers.